The present invention generally relates to improving signal quality in communication systems, and, more particularly, to methods for reducing distortion in communication systems employing continuously variable slope delta modulation in automobiles.
Wireless devices are increasingly employed in a wide variety of communication systems and applications. Examples of wireless-capable devices that have become increasingly popular include cellular telephones, pagers, home computers, laptop computers, and PDAs. A number of wireless communication protocols have developed to support communication among wireless devices, including, among others, 802.11 and BLUETOOTH. In addition to supporting communication among devices such as laptops, cellular phones, and PDAs, some of these protocols also provide the ability to wirelessly connect I/O devices, such as mice, keyboards, microphones, and speakers, to other devices, such as cellular phones and laptops.
The wireless connection of devices, such as microphones, to other wireless devices, such as cellular telephones, is particularly attractive because a wireless connection allows the user to be freed from often cumbersome and inconvenient wired connections to devices. Wireless connections also provide an additional advantage of allowing a single device, such as an I/O device, to be connected to a variety of other devices like cellular phones and computers, without requiring matching physical connecting structures on the devices to be connected. This helps to make the devices more interchangeable and flexible. For example, a BLUETOOTH microphone could be used by a variety of BLUETOOTH-capable phones without a need to worry about a matching physical connection. Wireless microphones can be particularly advantageous in an automotive environment where, for example, wireless microphones can be mounted in the vehicle (such as in a rearview mirror assembly), and used to connect to wireless devices (such as cellular phones) that are brought into the vehicle by various users.
While information communicated wirelessly can be in either analog or digital format, transmitting information in digital format does have advantages. For example, signal to noise ratios in digitally encoded signals typically do not vary with the transmission distance, and hardware for switching and processing digital signals can be less expensive than that required for analog systems. In addition, power consumption in digital systems can be lower than power consumption in analog systems. Because many of the signals to be transmitted and received in a digital network start out in analog form (for example, voice input to a microphone), it is often necessary to convert analog signals to digital form to enable effective wireless communication. Delta modulation is one method that can be used to convert analog signals, such as voice signals, into a digital pulse stream for digital communication purposes.
One form of delta modulation that has found use in wireless communication applications is Continuously Variable Slope Delta (CVSD) modulation. CVSD modulation is a method for coding speech or other waveforms as a pulse stream, and is often used in BLUETOOTH applications. This relatively low bit-rate method is attractive in part because of its relative simplicity and low implementation cost. When used in conjunction with wireless communication, a CVSD encoder is often used to convert an analog speech input signal into a digital pulse stream output. The output digital pulse stream can then be wirelessly transmitted to a CVSD decoder in a receiver, which converts the digital pulse stream to an analog signal for use by the receiver (i.e., for playback on a loudspeaker), or subsequent transmission by a cellular telephone transceiver.
Referring to FIGS. 1 and 2, operation of a typical CVSD modulation scheme will now be discussed. FIG. 1 illustrates a CVSD encoder 11 having a comparator 13 for comparing an input source signal with an integrated signal from an integrator 22. If the voltage of the input source signal exceeds the voltage from integrator 22, comparator 13 will output a digital “1.” If the voltage of the input source signal is less than the voltage from integrator 22, comparator 13 will output a digital “0.” The output signal from comparator 13 is provided to shift register 14, which stores the current output value of comparator 13 along with the previous 2 values of comparator 13. The output signal from comparator 13 is also provided as a digital signal output from the CVSD encoder, which can then be provided to a transmitter for transmission to a receiver. Over time, this digital signal output takes the form of a pulse stream of “1's” and “0's.” The output signal from comparator 13 is also provided to a pulse amplitude modulator 20, which applies a positive or negative charge to integrator 22 depending on whether the output signal from comparator 13 is a “1” (resulting in a positive charge being applied to integrator 22) or a “0” (resulting in a negative charge being applied to integrator 22). Logic 16 monitors the values of the bits in shift register 14 and performs an “overload” algorithm. The purpose of the algorithm is to determine when the CVSD encoder circuitry is in a slew rate limited condition. A slew rate limited condition occurs when the difference between the integrated signal from integrator 22 and the source signal is so great that integrator 22 is unable to reach the level of the source signal within a certain time period. A slew rate limited condition is indicated when the values in shift register 14 are equal (either all “1's” or “0's”).
When the algorithm performed by logic 16 determines that the values in shift register 14 are equal, it provides a signal to syllabic filter 18 indicating that a slew rate limited condition exists. Syllabic filter 18 then sends a signal to pulse amplitude modulator 20 to either increase or decrease the amount of current being applied to integrator 22, depending on whether the integrated voltage needs to increase or decrease more quickly to match the source signal. In addition, syllabic filter 18 determines how long the circuitry has been in a slew rate limited condition, and sends signals to pulse amplitude modulator 20 to increase the rate of increase/decrease in current applied to integrator 22. Syllabic filter 18 will cause pulse amplitude modulator 20 to continue to increase/decrease the rate of change in current applied to pulse amplitude modulator 22 until logic 16 determines that a slew rate limited condition no longer exists.
The effect of the operations discussed above is to provide an output of integrator 22 that tracks the voltage of the source signal. These operations also result in the digital output signal of CVSD encoder 11 being a digital pulse stream indicative of whether the source input voltage is increasing (a stream of 1s), decreasing (a stream of 0s), or staying the same (stream of alternating 1s and 0s).
FIG. 2 illustrates a typical CVSD decoder 12. Much of the circuitry in decoder 12 is identical to the circuitry in encoder 11. In operation, decoder 12 receives a digital pulse stream output from CVSD encoder 11, and provides it to a pulse amplitude modulator 20. As noted above, pulse amplitude modulator 20 applies either a negative or positive charge to integrator 22, depending on the whether the input is a digital “1” (positive charge applied to integrator 22), or a digital “0” (negative charge applied to integrator 22). Decoder 12 also contains a shift register 14, logic 16, and syllabic filter 18, for determining if the signal is in a slew rate limited condition, and for increasing/decreasing the rate of change of the charge applied to integrator 22 based on the existence and duration of a slew rate limited condition. By using elements in the decoder circuitry that are identical to the elements in the transmit circuitry, the decoder circuitry is able to convert the digital pulse stream into an analog output signal that tracks the input source signal of the encoder 11.
While the CVSD encoding and decoding scheme discussed above and shown in FIGS. 1 and 2 can be a practical and cost-effective tool for communicating voice and other signals, it does have limitations. For example, when transmitting audio, such as voice signals, CVSD encoders can introduce subharmonics and other nonlinearities into the encoded audio. The CVSD encoder can also introduce level-dependent frequency response variations into the signal. CVSD modulation also has a fairly limited dynamic range. At higher signal levels, distortion and slew rate limiting can become severe. At lower signal levels, the noise floor can become an issue. These and other limitations can reduce the intelligibility of the signals communicated in a CVSD modulation scheme, and can also reduce the effectiveness of software or devices utilizing or interpreting CVSD-processed signals. For example, the effectiveness of speech recognition software operating on signals received through a BLUETOOTH link can be reduced by these limitations.
The inventors have recognized a need to provide a method for reducing nonlinear distortion and improving the frequency response of systems employing CVSD modulation, and for optimizing the dynamic range associated with systems employing CVSD modulation. The inventors have also recognized a need to provide the above-noted advantages while providing for an improved CVSD encoder that is compatible with the Bluetooth specification, and with existing typical CVSD decoders, such as, for example CVSD decoders complying with the Bluetooth specification or the MIL-STD-188-113 specification. This allows for improved performance of existing CVSD decoders and applications employing existing CVSD decoders. More specifically, this allows for improved performance of millions of existing Bluetooth devices already in the field when those existing Bluetooth devices are used in conjunction with the improved encoder.